Jokiharju was drafted by the Blackhawks in the first round (29th overall) of the 2017 National Hockey League Draft. The 18-year-old recently finished his second season with the Portland Winterhawks of the Western Hockey League where he was named to the Western Conference Second All-Star team and posted a career-high 71 points, including career-highs in goals (12) and assists (59) in 63 games. His 59 helpers shared second and his 71 points shared third among all WHL defensemen. Jokiharju's 71 points were the most by a Winterhawks' defenseman since the 1993-94 season.

The Oulu, Finland, native helped Portland reach the second round of the 2018 WHL Playoffs, recording eight points (3G, 5A) in 12 games. During the 2017-18 regular season, Jokiharju recorded 20 multi-point games with the Winterhawks, including three four-point efforts and a nine-game point streak (2G, 15A) from November 11-29, 2017. In two seasons with Portland, he accumulated 21 goals and 98 assists in 134 regular-season contests.

Internationally, Jokiharju represented his country at the 2018 World Junior Championship, notching four points (2G, 2A) in five games. He also helped Finland to a Gold Medal at the 2016 U18 World Junior Championship, compiling three assists in seven games during the tournament.